Contrast-enhanced ultrasound imaging is a commonly used medical imaging modality. Most if not all ultrasound contrast agents (UCA) are gas-filled microspheres that are useful in enhancing ultrasound signal. One such UCA is activated DEFINITY® comprising perflutren lipid microspheres (i.e., perflutren gas encapsulated in lipid microspheres). DEFINITY® formulation is packaged in a vial comprising lipids in an aqueous suspension with perflutren gas in the headspace. Prior to use, DEFINITY® is activated by shaking the vial vigorously, thereby forming lipid microspheres comprising perflutren gas suspended in an aqueous liquid. Proper activation ensures that the microspheres formed are of the appropriate size and concentration to be both diagnostically effective and safe for the subject. Due to the importance of proper size and concentration, activation should optimally be performed in a manner that minimizes the potential for human error.